ACVR1 and plasma cell myeloma: For example, in human myeloma and liver carcinoma cells, Olsen et al [60] showed that ectopic Activin can phosphorylate SMAD1/5/8 via ALK2 (ActivinR1), and knockdown of the type II receptor BMPR2 increases pSMAD1/5/8 via BMP6, BMP7, and BMP9 and ALK2.